Sudoku Made and maintained by Brian Hamilton and Karan Hamilton.
Brian builds the playable puzzle tools, while Karan helps test clarity, difficulty feel, and the player experience.
Playable puzzles and controls are tested in the browser for rule accuracy, usability, and a clear solving experience. See how we make and test puzzles, our editorial standards, or report a problem.
What is Skyscraper Sudoku?
Skyscraper Sudoku is sudoku with a skyline. Fill the grid so that every row, column and 3×3 box holds 1 to 9 once, as usual, and read each digit as a building that many storeys tall. The numbers round the edge are clues:
- A clue says how many buildings you can see looking into the grid from that spot, along its row or column.
- A taller building hides every shorter one behind it, so you see a building only if it is taller than everything in front of it.
- Where no clue is shown, that view tells you nothing; Easy shows all 36 clues, the harder levels fewer.
So the row 3 1 5 2 9 4 6 7 8, read from the left, shows the 3, the 5 and the 9: its clue on that side is 3. From the right you see the 8 and the 9, so that clue is 2. The 9 is seen from both ends of every line, and nothing behind it ever is.
Take away the boxes and shrink the grid, and the clue makes a puzzle by itself: every row and column holds each height once, and usually nothing but the numbers round the edge is given. That one is usually sold as Skyscrapers or Towers.
What does one clue tell you?
How close to the edge the tall buildings can stand. If the first square held a 7, only the 8 and the 9 could be seen behind it, so no more than three buildings in all. Turn that round and a clue of k means the first square holds at most 10 − k, the next square in, one more than that, and so on; the 9 is never nearer than the kth square.
The two ends of the scale say the most. A 1 means the 9 stands right at that edge, hiding the whole line. A 2 keeps the 9 out of the first square. A 9 would mean the line runs 1 to 9 in order, and an 8 nearly so, which is why neither turned up in 4,320 clues on 160 of our boards; a 3 was the commonest, and 1s were about one clue in nine.
Where does the 9 go?
Between its two clues. The 9 is the last building seen from either end, so a line with a clue at each end fences it in: counting from the start, the 9 stands in square a or further in, and counting from the other end, square b or further in. The two clues of a line never add up to more than 10, and the stretch they leave is 11 − a − b squares long, so clues that add up to 10 place the 9 outright. Otherwise the rows, columns and boxes pick among the squares left:
Once a line’s 9 is in, read its clues again: each one now only concerns the squares between it and the 9. A clue of 2 with the 9 in the fifth square, for instance, means just one of the four buildings in front of the 9 is seen — the first, which is always seen — so the first square holds the tallest of those four.
Were the boards on this page fair?
Most were not. The old generator filled a grid, showed a random share of the clues and a random set of digits, and never checked what it had dealt. We dealt 40 boards at every level and counted their answers with a separate solver: 111 of 160 had more than one — 8 of the Easy boards, 25 Medium, 38 Hard and all 40 Expert, where 9 clues and no digits left at least half the boards with 200 answers or more. And on 7 of the Easy boards the clues did nothing at all: the 35 given digits settled the grid by themselves.
A board with two answers is worse than hard. The page marks a digit wrong when it differs from the one answer it stored, so a digit that broke no rule could still cost you a mistake, and no reasoning could finish the board. The generator was rewritten in September 2026. It now takes a digit out only while the level’s own steps still solve the board with the clues shown, which proves the board has exactly one answer, and it keeps digging until the digits alone no longer settle it:
How do the four levels differ?
By how many clues are shown and how many digits are given, and so by how much the clues have to do. We dealt 40 fresh boards per level and read every clue shown until nothing more changed, before placing a single digit:
| Level | Clues shown | Given digits | Squares the clues settle | Need locked candidates |
|---|---|---|---|---|
| Easy | 36 | 30 | 50 of 51 | 0 of 40 |
| Medium | 28 | 20 | 43 of 61 | 0 of 40 |
| Hard | 24 | 16 | 15 of 65 | 4 of 40 |
| Expert | 20 | 13 | 7 of 68 | 11 of 40 |
40 boards per level from this page’s generator, fixed seed; medians. Expert digs as far as its steps allow, from 12 to 17 digits. “Squares the clues settle” is our solver reading every clue exactly — trying every order of a line’s digits that its pencil marks allow — which is more than the two rules above can do by hand. Our solver also knows hidden singles, naked pairs and triples and the X-Wing; not one of the 160 boards needed anything past locked candidates.
On Easy the clues very nearly solve the board by themselves. On Expert they settle a handful of squares, and the rest comes from working the clues and the sudoku rules against each other, with locked candidates on more than a quarter of the boards. Ignore the clues and our solver stalls on every board at every level.
A digit comes out only while the level’s steps — clue reading and singles on Easy and Medium, plus locked candidates on Hard, plus naked pairs on Expert — still fill the grid, and each step only strikes digits that cannot go where they are struck. We re-counted all 160 boards with a separate counter written for the purpose: every one had exactly one answer, the stored one, and without its clues every one had more.
Which level should you play?
Start on Easy if the clues are new to you. All 36 are shown and 30 digits are given, so every line can be read from both ends and the rules above go a long way. The page opens on Medium, which hides eight clues and gives ten fewer digits.
Hard and Expert show fewer clues and give fewer digits, so the clues settle far less on their own — 15 and 7 squares — and the rest comes from playing them against the rows, columns and boxes. Every board still solves without guessing, and a new one appears in well under a tenth of a second at the median.
A solving order that works here
- Start with the 1s. Each one puts a 9 at the edge, and that 9 leaves its row, column and box.
- Then fence in the 9s. On every line with two clues, the 9 stands between square a from one end and square b from the other.
- Write in the limits. Next to a clue of k, the first square holds at most 10 − k, and each square further in one more. In your pencil marks, that strikes the tall digits near big clues.
- Re-read a line whenever it fills in. With a few heights placed, a clue can leave only one order for the rest.
- Then play sudoku. Singles in rows, columns and boxes, and on Hard and Expert, locked candidates.
What do the buttons do?
Select a square, then a digit; the page opens on Medium. Selecting a square highlights its row, its column and its box, and a wrong digit is marked the moment it goes in. Under the grid:
- Notes turns pencil marks on and off. A correct digit clears itself from the notes of its row, its column and its box; a wrong one leaves the other squares’ notes alone.
- Erase clears the selected square, and pressing the same digit twice does the same.
- Undo takes back the last change to a square, though not the notes a correct digit cleared elsewhere, and not the Mistakes count.
- Hint fills one square with its answer, picked at random rather than the square you could work out next.
- Solve asks first, then fills the grid and stops the timer.
- New Game deals another board at the level you are on; the level buttons deal one at theirs.
From a keyboard: 1–9, the arrow keys, Backspace or Delete and Ctrl+Z. The board is saved in this browser after every move, so it is waiting when you come back.
Where to go next
Sandwich Sudoku also puts its clues outside the grid, as the sum of the digits between each line’s 1 and 9, and Battleship Sudoku as a count of ship squares. Greater Than Sudoku keeps the comparing but moves it inside, between neighbouring squares.